diff --git a/websocket-sharp/Net/Cookie.cs b/websocket-sharp/Net/Cookie.cs
index 31d64137..bf5a79c1 100644
--- a/websocket-sharp/Net/Cookie.cs
+++ b/websocket-sharp/Net/Cookie.cs
@@ -795,7 +795,7 @@ namespace WebSocketSharp.Net
       if (!_domain.IsNullOrEmpty ())
         buff.AppendFormat ("; Domain={0}", _domain);
 
-      if (!_port.IsNullOrEmpty ()) {
+      if (_port != null) {
         if (_port != "\"\"")
           buff.AppendFormat ("; Port={0}", _port);
         else